Tax interacts with the cellular CREB protein and facilitates the binding of the coactivator CBP, forming a multimeric complex on the CRE-like sites in the HTLV-I promoter. The trimeric complex is believed to recruit additional regulatory proteins to the HTLV-I LTR. Recent studies have shown that the p300/CBP binding protein-associated factor (PCAF) is involved in transcriptional activation. PCAF activity has been shown strongly associated with histone acetyltransferase (HAT) activity. In a recent report, we provide evidence for a HAT-independent transcription function that is activated in the presence of the HTLV-I Tax protein. In vitro and in vivo GST-Tax pull down and co-immunoprecipitation experiments demonstrate that there is a direct interaction between Tax and PCAF, independent of p300/CBP. PCAF can be recruited to the HTLV-I TRE site in the presence of Tax and PCAF cooperates with Tax in vivo to activate transcription from the HTLV-I LTR over 10-fold. Point mutations at Tax amino acid 318 (TaxS318A) or 319-320 (Tax M47), which have decreased or no activity on the HTLV-I promoter, are defective for PCAF binding. Strikingly, the ability of PCAF to stimulate Tax transactivation is not solely dependent upon the PCAF HAT domain. Two independent PCAF HAT mutants, which knock out acetyltransferase enzyme activity, activate Tax transactivation to approximately the same level as wild-type PCAF. In contrast, p300 stimulation of Tax transactivation is HAT-dependent. These studies provide experimental evidence that PCAF contains a coactivator transcription function independent of the HAT activity on the viral LTR.